The present invention generally relates to a printing system and a method of controlling the printing system. More particularly, the present invention relates to a printing system and method which controls paper conveyance using a step motor and automatically detects the size of paper fed into the system before performing the printing operation.
In a conventional printing system, such as a drum printer or capstan printer, print media is conveyed (e.g., via a step motor) to a print head so that input images can be printed on the paper.
FIG. 1 is a block diagram of a conventional printing system for controlling the paper conveyance in which the capstan-driving technique is employed. The printing system includes: paper cassette 12 on which sheets of paper are stacked; paper feeding motor 14 which conveys the paper loaded on paper cassette 12; paper driving portion which consists of platen 22, capstan roller 18, and step motor 24; and a paper output portion with paper output motor 26; and central processing unit (CPU) 16 that controls paper feeding motor 14, step motor 24, and paper output motor 26 by analyzing the paper selection information from paper cassette 12. In FIG. 1, reference numeral 20 denotes a pinch roller which rotates with the turning of the capstan roller 18.
The conventional printing system shown in FIG. 1 receives information as to the size of the paper loaded in paper cassette 12. This paper size information is entered by a user through the paper selector switch on paper cassette 12. A sensor installed in the printing system senses the paper size information and transmits it to CPU 16.
CPU 16 stores the paper size information transmitted from the sensor in its internal memory and controls paper feeding by computing the paper conveyance distance from the paper size information in response to a command to start printing. Typical paper sizes include Letter-, A4- and OA4(oversize A4)-sized sheets of paper, and the CPU 16 stores in a memory, information about the paper conveyance distances X, Y and Z according to the paper sizes/types.
When the paper size information is transmitted from the sensor under the present condition, the CPU 16 computes a paper feeding distance during printing according to the paper size, and controls paper feed motor 14 to feed the paper into the system. When the paper selector switch on the paper cassette is adjusted for Letter, A4 or OA4 paper sizes, CPU 16 selects the corresponding paper conveyance distance of "X", "Y" or "Z", and controls the amount of paper to be fed into the system by the selected paper conveyance distance. Accordingly, when the paper feeding is completed, CPU 16 assigns a proper print position to the paper according to the paper size information, and allows the printing operation. The amount of paper fed into the system can be regulated by controlling paper feeding motor 14.
Conventional printing systems, which regulate the amount of paper fed into the system according to paper size information from the paper size selector switch have many problems, as set forth in the examples below. The printing system to which the present invention can be most effectively applied will be described, while reviewing these problems. That is, the following description concerns a sublimation color printer which employs the sublimation printing technique and capstan driving technique and uses typical paper sizes of A4--297.times.210 mm; Letter--279.times.216 mm; and OA4--337.times.243 mm. FIGS. 2A, 2B, 2C, 3A, 3B and 3C are views for describing the paper feeding problems of conventional techniques.
FIGS. 2A, 2B and 2C show the sizes of paper and the differences in paper conveyance distance. FIG. 2A depicts the length of each paper, and FIG. 2B shows the OA4-sized paper location at the initial stage of printing. FIG. 2C depicts each location of the OA4-sized paper and other print media after the paper feeding.
FIGS. 3A, 3B and 3C show image outputs on OA4-, A4- and Letter-sized sheets of paper. FIG. 3A describes printing areas of OA4-, A4- and Letter-sized sheets of paper during the normal printing operation, and FIG. 3B is a view for describing a printing area of paper whose size is smaller than the paper selection information in a sublimation color printer. FIG. 3C illustrates printing areas of paper whose size is larger than the paper selection information.
Some of the paper feeding problems of conventional techniques are now described with reference to FIGS. 2A, 2B, 2C, 3A, 3B and 3C. If the size of paper loaded on the paper cassette 12 does not correspond to the paper size information preset in the paper cassette 12, i.e. if the actual size of paper loaded thereon is either smaller or larger than the paper size information of the paper cassette 12, the following problems may occur.
If the paper loaded in paper cassette 12 is Letter- or A4-sized while the paper selector switch of the paper cassette 12 is adjusted for OA4 paper size, or if the paper loaded on the paper cassette 12 is Letter-sized while the paper selector switch of paper cassette 12 is adjusted for A4 paper size, the capstan roller 18 and the pinch roller 20 are still rotated by the step motor 24 even after the paper is delivered to them. This causes the tail of the paper to slip out of the two rollers, and the printing system cannot operate normally.
For example, when the paper loaded on paper cassette 12 is Letter-sized while the paper selector switch of the paper cassette 12 is adjusted for OA4 paper size, the paper fails to be inserted between the capstan roller 18 and the pinch roller 20 during paper feeding so CPU 16 cannot control the printing operation. This problem may also occur when A4-sized paper is loaded in paper cassette 12. Provided the paper conveyance distances X, Y, Z with respect to letter-, A4- and OA4-sized sheets of paper are 269 mm, 287 mm and 327 mm which are smaller than their actual paper lengths of 279 mm, 297 mm and 337 mm by 10 mm, the differences between "Z" and "X", "Z" and "Y", and "Y" and "Z" are 58 mm, 40 mm, 18 mm. Thus, in the above cases, the paper fails to reach capstan roller 18 and pinch roller 20.
The above description is depicted in FIGS. 2A, 2B and 2C. FIG. 2A shows the actual lengths of Letter-, A4- and OA4-sized sheets of paper, and FIG. 2B depicts the initial stage of paper feeding in which the OA4-sized paper is inserted between capstan roller 18 and pinch roller 20. The dimension "Z", of FIG. 2B, denotes a distance by which the paper is moved by capstan roller 18. FIG. 2C illustrates the location of the paper that has been moved by distance "Z". As shown in FIG. 2C, the tail of the OA4-sized paper is interposed between the capstan roller 18 and the pinch roller 20 by about 10 mm, and the A4- and Letter-sized sheets of paper fail to remain inserted between the capstan roller 18 and the pinch roller 20 so the printing operation cannot perform normally.
Under these circumstances, even if printing is carried out by preventing the paper from slipping out of the capstan roller 18 and the pinch roller 20 using some type of control device, each paper may have different printing dimensions, and input images cannot be printed without proper alignment.
FIGS. 3A, 3B and 3C show image outputs on OA4-, A4- and Letter-sized sheets of paper during the printing operation under the above condition. FIG. 3A describes printing areas of OA4-, A4- and Letter-sized sheets of paper during the normal printing operation wherein the paper loaded in paper cassette 12 corresponds to the paper size information of paper cassette 12.
In FIG. 3A, L1 and L2 are the differences in image length between OA4 and A4 sheets and between OA4 and Letter sheets. FIG. 3B shows cases where the paper loaded in paper cassette 12 does not correspond to the paper size information of paper cassette 12. In the cases of FIG. 3B, the paper loaded on the paper cassette 12 is either Letter- or A4-sized where the paper selector switch of paper cassette 12 is adjusted for OA4 paper size, or the paper loaded in the paper cassette 12 is Letter-sized while the paper selector switch of paper cassette 12 is adjusted for A4 paper size. In these cases, since the paper size is smaller than the size of an image to be printed, the print length is only C or D which are shorter than the overall image size S. The printing continues for the distances "L1" or "L2", so that the tail of the paper fails to be inserted between the capstan roller 18 and the pinch roller 20. At this point, because the paper selector switch is adjusted for OA4 paper size, the thermal printing head (TPH) keeps on producing heat. Accordingly, the heat produced by the TPH is directly applied to platen 22, damaging platen 22 which causes printing errors. When using a sublimation color printer, this entails a loss on its color ink ribbon by an amount for one-sheet printing. Moreover, the heat from the TPH may reduce the lives of platen 22 and the TPH.
If the paper loaded in paper cassette 12 is larger than the paper selection information of paper cassette 12, i.e. if the paper selector switch is adjusted for Letter paper size and the paper in paper cassette 12 is OA4-sized one, this results in a waste of paper. FIG. 3C illustrates printing areas of OA4-, A4- and Letter-sized sheets of paper when the printing operation is carried out under the above-mentioned condition. As shown in FIG. 3C, even though input images can be printed on the paper, the paper size is too large for the output images, which causes the waste of paper and prevents the input images from being printed on the middle of the paper. Since print media used for the sublimation color printer is very expensive, and it is not easy for users to pay for its upkeep and repair. The heat produced by the TPH affects the durability of the TPH and capstan roller reducing the reliability of the printing system.